Mixed signal integrated circuits require a steady power supply. When the voltage level of the supply voltage drops below an operating level, the integrated circuit will malfunction. Thus, it is important to monitor the voltage level of the supply voltage so that the device may be deactivated prior to malfunctioning.
Commonly a power-on-reset (POR) circuit is used to monitor the supply voltage and to predict when the voltage level of the supply voltage is adequate to assure proper operation of the system components of the mixed signal circuit. The POR circuit provides a signal that deactivates the mixed signal circuit when the supply voltage is below a threshold level.
FIG. 1 is a diagram illustrating a conventional integrated mixed signal system 10, including a POR circuit 12, a logic circuit 14, and a supply voltage Vdd powering four analog circuits 16, 18, 20, and 22 as well as POR circuit 12. Analog circuits 16-22 may be voltage references, system clocks, amplifiers and other such system components. It is understood, of course, that the analog circuits and logic circuit shown in FIG. 1 are illustrative and that mixed signal system 10 may have many additional analog circuits and logic circuits powered by supply voltage Vdd.
Analog circuits 16-22 receive and transmit analog signals "a" from external and/or internal devices (not shown), and provide and receive digital signals d to logic circuit 14. Logic circuit 14 provides a digital signal d to an external device (not shown) such as another logic circuit or an output interface.
As shown in FIG. 1, POR circuit 12 is connected directly to the supply voltage Vdd. The POR circuit 12 provides a digital signal d to logic circuit 14, which is a "deactivate signal" indicating when supply voltage Vdd is below a threshold voltage level. The threshold voltage level is the minimum threshold for proper operation of all system components. Logic circuit 14 is deactivated when it receives the deactivate signal from POR circuit 12 so that mixed signal system 10 does not operate when the voltage level of supply voltage Vdd is below the threshold voltage level. Of course, POR circuit 12 may alternatively produce an "activate signal" to activate logic circuit 14 when supply voltage Vdd is adequate to power all system components of mixed signal circuit 10.
As shown in FIG. 1, POR circuit 12 is external to logic circuit 14 and analog circuits 16-22. Because POR circuit 12 directly monitors supply voltage Vdd without input from the actual system components, i.e., analog circuits 16-22, POR circuit 12 must predict when supply voltage Vdd is inadequate to power these system components. Thus, POR circuit 12 uses a preselected voltage range within which supply voltage Vdd is permitted to vary. When POR circuit 12 detects the voltage level of supply voltage Vdd varying beyond the preselected voltage range, the deactivate signal is triggered.
Because POR circuit 12 does not receive input from other system components, the voltage range within which supply voltage Vdd may vary is preselected such that all system components are assured to have sufficient power to operate properly. However, the appropriate voltage range for some of the components of mixed signal system 10 may be different from other components, particularly where a component is more temperature dependent than other components. Further, the appropriate voltage range may vary from one mixed signal system to the next due to process variances. Thus, to assure that the deactivate signal produced by POR circuit 12 occurs at a voltage level that is sufficient to power all system components, a usable voltage range is typically sacrificed. In other words, the minimum supply voltage Vdd level permitted by POR circuit 12 is greater than is actually necessary to power the system components. However, as technological improvements permit the magnitudes of supply voltages to decrease, it is increasingly important to minimize sacrificed voltage ranges.
Moreover, to accurately monitor supply voltage Vdd, POR circuit 12 generally uses a voltage generator, such as a bandgap voltage reference circuit, to produce an accurate temperature independent reference voltage to compare with supply voltage Vdd. However, where one of analog circuits 16-22 is a bandgap voltage reference circuit or similar temperature independent voltage supply the use of a second bandgap voltage reference in POR circuit 12 is redundant. Thus, the cost and complexity of mixed signal system 10 is increased while valuable real estate on the integrated circuit is wasted.